Thanks to almost everyone that has posted helpful and understanding posts, it really is very much appreciated as I have no experience of this kind of thing, hard to know what to do for the best.
Dn is fed, is clean, has lots of things bought for her by both her mum & my mum, so not neglected in that sense, it's emotional neglect I think.
When in my company I make sure she has a huge hug and I ask her stuff about her life, school/friends/home etc, she is lovely & smiley with me, then quite quickly she becomes rude and argumentative with my mum & her mum over very little. I have asked her to no be so rude and changed the subject but it's constant with them.
I am adamant that I will maintain a positive and friendly relationship with her, if I tackle my sister, I know she will withdraw contact with me. I can't risk that.
Dn knows I love her, absolutely, I find her behavior towards others very distressing. But I exploit her good nature towards me fully as a foot in the door to her well being.
She has told a terrible lie about my son in the past, I will not put him into a position where she can do this a again. She has very little contact with my kids, maybe twice a year under constant supervision, they do not get along anyway. She is very antagonistic and deliberately argumentative with him, he doesn't know how to take her. They are the same age. Best to separate them really.
My sister is vague about things when I ask her, almost sleep walking through it all. Happy for any one to take dn off her hands.
Would camhs have anything to do with me if I rang and asked to speak to someone about dn? Confidentiality and all that?? I couldn't sleep with worry and felt permanently. sick when I was involved last time, it was making me ill. But want to do something to help.